For some of you who are apalled at the human race and throwing rocks, consider some things:
First, what about other less-than-stellar behaviors? Like spitting from a Top Spin when it's hanging you straight down (which invariably lands on other riders). Or, spitting/tossing coins/gum from cable-car type rides. Although not quite as grave in scope as tossing a rock at a moving roller coaster, teens don't view it as such. They view it as a small act of mischief that will make someone else slightly mad. Not a serious action that could cause severe injury or even death.
Most likely, the culprits of said rock-hurling are in their teens. Teenagers don't (always) have an encompassing understanding of their actions. That's why they are considered "minors" and need permission slips from parents who (theoretically) are thinking properly. A teenager thinks "gee, this will be funny to throw rocks at the coaster as it passes. The rock will be a little annoyance, and this will be a cute little prank". They don't realize that the coaster is going 60mph and will do serious damage.
Case in point (on not grasping the entirety of the situation): Either 1 or 2 years ago (I forget exactly) during the winter, a 14-year-old decided to throw a chunk of ice from an overpass onto a highway in my area. The kid thought that it would either shatter on the road in front of a car, or hit and bounce off a windshield, causing a little mischief and possibly a small fender bender, but nothing serious. What happened was that the ice chunk shattered and went through the windshield of a minivan, killing the poor woman who was in the passenger seat (and assorted cuts/scrapes from glass to the driver).
As for the park and their liability, I think that 3.6million is a bit steep.. however I *do* feel that they are responsible as well. Was there even so much as a sign prohibiting objects from being thrown in that area? What about surveillance cameras (if nothing else, they'd be able to find the person who threw the rock) or, as mentioned above, a security guard or net.